Free-space channels suffer losses due to beam spreading (i.e. diffraction or geometrical losses), absorption, and scattering [1]}. Turbulence will also cause the beam to both wander and distort [2]}, [3]}, reducing the coupling of the modes to the collection device. Sway of the transmitters and receivers together with additional pointing errors also adds to loss. These effects could be mitigated by active correction methods [4]}. Other source of loss are imperfect mode coupling into the detector, detector efficiencies, and losses within the transmitter and receivers optics.
